It has long been known that visible light such as white light is obtained from a combination of phosphors which absorb visible light in a short-wavelength region such as blue light and ultraviolet light and changes the wavelength to visible light of a longer wavelength such as red and green light.
In particular, light-emitting elements emitting visible light such as white light, which is composed by combining a light-emitting semiconductor element, e.g., gallium nitride (GaN) blue LED as a light source of visible light in a short-wavelength region and ultraviolet light and a phosphor as a wavelength-conversion material, have characteristics of low power consumption and long operating life, and are recently attracting attention as light-emitting sources for visual display units and lighting systems.
In such light emitting elements, for example, a phosphor as a conversion material absorbs visible light in a blue light region emitted from a GaN blue LED and emits yellow light, which is further mixed with the blue light from the LED which is not absorbed by the phosphor to be white light (cf. Patent Document 1 below).
The yellow phosphor, however, has a problem that its brightness lowers when temperature rises.
Moreover, as stable phosphors when temperature rises, nitride phosphors and oxynitride phosphors are proposed (cf. Patent Documents 2 and 3 below). Of the documents, there is a description in Patent Document 3, for example, that a phosphor of a composition of an alkaline earth element (AE):silicon (Si):oxygen (O):nitrogen (N)=1:2:2:2 has the good property.
The substance has a crystal skeleton of a combination of SiO4 tetrahedrons and SiN4 tetrahedrons. The phosphors of the structure still do not have sufficient emission brightness and improvement has been required.
Patent Document 1: JP 10-242513 A (p. 2)
Patent Document 2: JP 2004-134805 A (pp. 2, 6 and 8)
Patent Document 3: JP 2004-277547 A (pp. 2 and 13)